An apparatus having the general construction mentioned above is already known, for example, from French Pat. No. 1 444 213, in which the die-carrier and a cylinder in which it is internally guided are associated with each other by a finger sliding in a helical groove, this coupling system allows the tap-carrier to find a suitable longitudinal position, and then communicates a force in the same direction together with a rotary couple, but it needs to be designed for repetitive working conditions which are known in advance. Consequently, it is less well adapted to tapping holes of different diameters or to tapping holes of the same diameter but presented in parts of different materials.
Attempts have been made to give the tapping head a facility for adaptation by disposing a longitudinal spring between the cylinder and a tap-carrier which is angularly driven thereby. In addition to the numerous spring breakages which have been observed using these means, mention must also be made of the drawback which results from the impossibility of adjusting the force: neither its initial value nor its subsequent evolution can be adjusted.
Further, when using prior methods and apparatuses, dispersions in the useful height of the thread provided or else modifications in the shape of the initial threads are observed; these defects can be considered as secondary only if the parts to be threaded are solid or very thick, and they become far too important when they appear on parts formed from a metal strip where the height of the threaded portions cannot exceed the height which can be obtained by drawing out the metal to an extent which is necessarily limited.
Consequently, the invention proposes firstly to define a tapping method and secondly to provide an apparatus for implementing the method, whereby the user will benefit not only from an improved facility for adaptation to the dimensions of the holes to be machined and to the materials of the parts containing the holes, but also from better quality and improved manufacturing safety, accompanied by a higher manufacturing throughput.